This invention relates to structures for terminating the active area of a high voltage semiconductor device, and more specifically relates to novel, concentric and very low dose field rings which surround a device active area.
High voltage semiconductor devices such as power MOSFETs, IGBTs, diodes and Schottky diodes all have an active central area consisting of appropriate diffusions in the top surface of a semiconductor die. The main electrode of a device may be at a high potential relative to the die peripheral edge and the peripheral top surface surrounding the high potential active area.
To prevent breakdown of the silicon surface surrounding the active region due to high lateral electric fields, it is necessary to terminate the active region periphery. Conventional terminations include spaced concentric diffused rings, for example, P+ diffusions in an Nxe2x88x92 epitaxially grown body which receives the active diffusions, spaced conductive ring-shaped field plates which surround the active region, or a combination of ring-shaped field plates and ring-shaped diffusions.
The purpose of the diffused rings and field plates is to gradually spread the electric field laterally between the periphery of the active region and the die periphery to ensure that the lateral field is sufficiently low to prevent avalanche breakdown at the die-peripheral surface.
Prior art field plates and diffused rings require that large surface areas be employed for a given die. It is desirable to reduce the area required for the terminal structure since die cost is approximately proportional to the die area.
Field plates, for example, aluminum field plates take up considerable area because they have a given width and require spacing from one another. Further, field plates are subject to damage. For example, in a fast recovery diode device designed to withstand a reverse voltage of 600 volts, four metal rings have been used atop and in contact with underlying P+ diffusions in an Nxe2x88x92 epitaxially formed surface. These rings have about a 4 micron thickness, a radial width of about 35 microns, and a spacing of about 6 microns. Thus, a substantial area is required for the field plate structure in conventional structures.
Diffused P+ rings in an Nxe2x88x92 epitaxial surface are also commonly used. These rings commonly have a high doping concentration, for example, a concentration produced by ion implanting with a boron dose of 7E14 to 1E16 atoms/cm2 and diffused to a depth of about 6 microns. Six such concentric rings may be used with constant or graduated spacings of from about 15 to 25 microns each. Thus, a substantial amount of area is also required for this type of termination.
In accordance with the invention, very lightly doped concentric field rings are used for a high voltage device termination. More specifically, a boron implant dose (preferably from 2E12 to 5E13 cm2) is used to form each of a plurality of spaced concentric termination diffusions in an Nxe2x88x92 substrate.
Lower doses, such as xe2x80x9cresurfxe2x80x9d doses of 3E11 to 5E11 cm2 are disclosed in U.S. Pat. No. 4,750,028 for termination rings. These doses, however, are so low that they are very difficult to control. Further, these ultra low doses permit the diffusion ring to fully deplete which could possibly permit punch-through failure to the silicon surface.
The high concentration usually used for termination rings, of 7E14 to 1E16 ions/cm2 causes a high electric field at the silicon surface and requires additional area for disposal of space field plates between diffusions in order to achieve proper reliability. By contrast, the novel low dose rings of the invention can be very narrow (about 2 microns), which low dose rings tend to move the electric field into the bulk of the wafer and away from its surface.
Advantageously, the low dose rings of the present invention lower the electric field at the surface of the die. By contrast, the high electric field at the surface of the die of prior art diffused rings can cause premature surface breakdown and cause the characteristics of the device to be undesirably sensitive to surface preparation conditions.
Thus, the low dose rings of the present invention provide a reduced area termination for a given die and is more reliable than prior designs.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.